This study is undertaken to determine and compare the incidence of heparin-associated thrombocytopenia in patients on full dose and minidose procine intestinal mucosal heparin; to determine and compare the incidence of heparin-associated thrombosis in patients on full-dose and minidose procine intestinal mucosal heparin; and to investigate the mechanisms involved in the pathogenesis of heparin-associated thrombocytopenia. All patients admitted to the hospital who will receive minidose heparin will be considered eligible for the study and will form one arm of the study. Patients on or admitted to the medical services for full dose heparin will be eligible for the second arm of the study with the following exclusions: patients in shock, patients with diffuse intravascular coagulation, and patients with malignancy. All patients will receive procine intestinal mucosal heparin. The incidence of thrombocytopenia associated with administration of minidose heparin will be determined and compare statistically with the incidence of thrombocytopenia in patients on full dose heparin. Similarly, the incidence of heparin associated thrombosis in patients on minidose heparin will be determined and compared statistically with the incidence of thrombosis in patients on full dose heparin. The various platelet function tests in thrombocytopenic and non-thrombocytopenic control patients will be compared to assess the predictive value of these tests.